a. Field of the Invention
The invention relates to phase locked loops, and more particularly to phase comparators used in phase locked loops for reading recorded digital data.
b. Prior Art
In certain digital data recording and playback or reading schemes, the presence or absence of a signal, during a specified time period represents a data bit. Data encoding schemes such as FM and MFM, usually used in magnetic disk recording, rely on reading data by detecting pulses indicative of high-to-low or low-to-high transitions of encoded data during precisely timed intervals or bit cells. For example, ones may be represented by transitions in the center of a bit cell and zeroes may be represented by transitions at the leading boundary of a cell, except where a zero follows a one, in which case no transition is made. Thus, bit cell centers and boundaries must be known as precisely as possible. These bit cells may be defined by clocks external to the data being read, or may be defined by the data itself, or by clocking signals interspersed with the data.
In the latter situation timing signals have been supplied in the prior art by an oscillator in a phase locked loop whereby the oscillator, running at the highest encoding harmonic frequency, is synchronized to the data by the data itself. If, for example, the data encoding frequencies were 0.5f.sub.o, 0.067f.sub.o and f.sub.o a voltage controlled oscillator, VCO, in a phase locked loop is locked in frequency to the harmonic frequency, f.sub.o in this example, with the phase of the VCO waveforms having an electronically defined relation to the data, for example, by defining a "window" during which an interrogation for a data signal is made.
For purposes of providing noise immunity it is known that a VCO can be designed to put out a masking signal which provides an indication of when the VCO is "safe" and "unsafe" for synchronization. For example, when the VCO output approaches a zero crossing, it is a convenient or safe time for synchronization since zero crossings are easy to detect. The remaining time may be regarded as unsafe. Moreover, associated read systems circuits may require that the VCO not be disturbed except during a predetermined safe interval during a VCO cycle. The present invention deals with phase locked loops in which the VCO has such a masking signal associated with it.
When a system is reading data, signal loss or drop-out is sometimes experienced because of recording errors, defects in the recording medium or the data transmission path or perhaps other causes. In any event, temporary loss of the data signal usually causes loss of synchronization between the local oscillator and the data signal, on its return. In those playback systems having synchronization data, sometimes known as preamble portions recorded with the data, recourse may be had to such preamble for re-establishing synchronization. However, it is desirable to re-establish or initially establish synchronization without recourse to a specially recorded data preamble.
In U.S. Pat. No. 4,080,576, Huber et al. teach use of a phase detector which operates in a non-harmonic mode for tracking preamble data pulses at a lower, non-harmonic frequency for initial lock-on and then switches operation to a harmonic mode for tracking non-preamble data pulses at a higher, harmonic frequency.
An object of the present invention is to provide a circuit for synchronizing a local oscillator with incoming data pulses for reading the data pulses without recourse to preamble data. Another object is to provide such a circuit with improved noise immunity.